NICMOS grism wavelength calibration
TIPS - April 16, 2009
N. Pirzkal
R. Bohlin
D. Thatte
TIPS - April 16, 2009
NICMOS Grism Mode
• 3 grisms
• NIC3
• Low resolution (200 angstrom/pixel)
• G096: 0.8μ < λ < 1.2μ
• G141: 1.1μ < λ < 1.9μ
• G206: 1.4μ < λ < 2.5μ
TIPS - April 16, 2009
Previous Calibration
• Mainly Cycle 7 and 8 programs. i.e. ~10 years ago
• Calibration done using PNe observations of VY-22 and HB12
λ = m ∗ ∆x + b
∆x = (x − x0 )
• No field dependence calibration
• No verification of repeatability
• Previous calibration done in column space
and not along the tilted trace
• No field dependence information
TIPS - April 16, 2009
m
b
G096
-0.00536
0.9487
G141
-0.00799
1.401
G206
-0.01152
2.045
PNe as calibrator in the NIR
• Bright emission lines
• Provides a few good lines in the range of each of the NIC3 grisms
• Some are blends, but these can be measured in smoothed reference spectrum
G096
G141
G206
TIPS - April 16, 2009
Program 11331 Strategy
• 2 PNe
• 4 positions
• 2 dithers
• Total of 16 independent
spectra
• Aim for ~4 lines per spectra
• HB12 and VY22 observed 4
months apart
TIPS - April 16, 2009
NICMOS Trace
• Measure positions of lines on the grism/slitless images
• Compute the equation of the trace:
(y-y0 ) = a1 ∗ (x − x0 ) + a2 ∗ (x − x0 )2 + ... + an ∗ (x − x0 )n
• NICMOS is linear within the measurement uncertainties
• Slope varies randomly (due to grism/filter positioning errors)
200
Grism
Slope (Deg)
150
G096
3.05 +/-0.19
100
G141
0.95 +/-0.20
50
G206
1.37 +/- 0.36
100
150
200
TIPS - April 16, 2009
Parametrizing the dispersion relation
TIPS - April 16, 2009
Allowing for field dependence
• m and b can be made to vary as a function of the position of the source (x,y) on
the detector
λ = m ∗ δl + b
λ = m(x0 , y0 ) ∗ δl + b(x0 , y0 )
m(x, y) = m0 + m1 ∗ x + m2 ∗ y + ...
b(x, y) = b0 + b1 ∗ x + b2 ∗ y + ...
0.943
!0.00563
!0.00564
0.942
!0.00565
0.941
200
!0.00566
!0.00567
200
0.940
0
0
100
100
100
100
200
200
0
0
TIPS - April 16, 2009
NICMOS grism variation over field
• Very small variation over the field
m
b
G096
-0.0056
+/- 0.3%
0.941
+/- 1%
G141
-0.0080
1.396
+/- 2% +/- 0.2%
G206
-0.0114
2.039
+/- 2% +/- 0.2%
• No significant change over a period of
4 months
• Once the varying spectral slope is
accounted, all 3 grisms are well
calibrated
• 1st order trace
• 1st order wavelength dispersion
• 1st order field dependence
TIPS - April 16, 2009
Example: G096 - Small field variation
200
150
0.004
m
0.002
0.000
200
!0.002
100
!0.004
0
100
100
50
200
0
60
80
100
120
140
160
180
200
150
0.002
b
0.001
0.000
200
!0.001
!0.002
100
0
100
100
50
200
0
60
80
100
120
140
160
180
New values confirm previous calibration
New
m
b
Old
G096
-0.0056
+/- 0.3%
0.941
+/- 1%
G096
-0.0054 0.9487
G141
-0.0080
1.396
+/- 2% +/- 0.2%
G141
-0.008
1.401
G206
-0.0114
2.039
+/- 2% +/- 0.2%
G206
-0.0115
2.045
TIPS - April 16, 2009
m
b
Putting it all together: Consistent solution
over all 3 grisms
TIPS - April 16, 2009
G096 - Old
TIPS - April 16, 2009
G096 - New
TIPS - April 16, 2009
G141 - Old
TIPS - April 16, 2009
G141 - New
TIPS - April 16, 2009
G206 - Old
TIPS - April 16, 2009
G206 - Old
TIPS - April 16, 2009
All 3 grisms - Old
TIPS - April 16, 2009
All 3 grisms - New
TIPS - April 16, 2009
Conclusion
• Wavelength calibration of all 3 NICMOS grism show no large change from
previous values (Cycle 7). (This is to be further tested after SM4)
• 16 independent observations, 4 months apart show no time variation
• New wavelength solution is consistent over all three grisms
• Calibration is done *along* the trace and not just by counting the number of
columns between source and pixel in the spectrum. This is how it is done for
ACS and WFC3
• Field dependence has been calibrated (but it is only a ~ 1-2% effect)